NIR spectroscopic observations were key to identifying He-core and CO-core WD mergers as the progenitors of dLHdC and RCB stars. The K-band spectra of these stars show anomalously strong \(^{12}\) C\(^{18}\) O first overtone bands in addition to the \(^{12}\) C\(^{16}\) O first overtone bands [1]}. The values of \(^{16}\) O/\(^{18}\) O in dLHDC and RCB stars cover a remarkably wide range (from 0.3 to 50), thus in all cases 1–3 orders of magnitude smaller than the solar value (\(\sim \) 500). The large amount of \(^{18}\) O is thought to be produced by partial helium burning in a thin shell around the core of the WD merger remnant, which is convectively dredged up to the surface [2]}, [3]}.
